Yes, humans are capable of the most horrible things.
As for her upbringing, what I found was that she was the younger of two sisters, and favored/spoiled by their mother. Their father died when she was young, and then her mother, who she was close with, died *in her arms* when she was a teenager. They say she never was the same after that.
Later, her sister was murdered, strangled to death on a deserted rural road. After Theresa was arrested for the murders of her daughters, the police re-opened that investigation, but determined Theresa didn’t have anything to do with her sister’s death.
There wasn’t any record of abuse of any kind, leading me to believe she was mentally ill — maybe a malignant narcissist, along with everything else— all along. But that’s just speculation, of course.